The invention relates generally to electromagnetic interference, and more specifically to a system for reducing magnetic interference propagated from an electronic device such as a personal computer.
Modern electronic devices that rely upon computerized circuits or digital logic typically conduct voltage signals within the circuits that change at very fast rates. Because digital logic is usually implemented in terms of ones and zeros, with ones being represented by some positive voltage such as 5 v or 3.3 v, and zeros being represented by zero voltage, such circuits must quickly change state from a positive voltage to zero voltage millions or billions of times per second to provide the capability to quickly calculate or process information.
These rapid changes in voltage state within such circuits can be viewed over time as approximating what is known as a square wave, wherein the voltage changes between states almost instantaneously. In practice, all circuits have limited bandwidth, and so a perfect square wave is not achieved. But, the approximate square wave that appears in most digital logic circuits changes state more quickly with faster circuits and switching times, and contains higher frequency components as it changes state more quickly.
These high frequency components can be easily radiated as electromagnetic fields from the electronic circuit, especially when the wavelength of the high frequency components is short enough to become a significant fraction of the conductive traces or wires that connect the circuit components to each other, which then become effective antennas. Digital signals are rich in spectral content, often containing significant energy over a wide bandwidth of the radio spectrum. These signals must be shielded to reduce emissions to certain levels before a digital electronic product can be sold according to requirements imposed by most countries, and so must be understood and controlled in the design process.
The high frequency currents that contribute to electromagnetic radiation from circuits will tend to travel through a circuit along a path of lowest inductance, which is as a practical matter the path of lowest overall impedance. The overall path typically includes various circuit traces on one or more layers of a printed circuit board, and conduction back to a ground potential through a relatively uniform ground plane layer of the circuit board. The loop formed by the circuit path contributes to both electrical and magnetic radiation, and is proportional to the current conducted through the loop.
What is desired is a system and method for reducing the electromagnetic emissions radiated from a digital electronic device.
The present invention provides a system and method incorporating one or more magnets affixed in proximity to a ground plane of a printed circuit board such that the magnetic flux of the magnet is operable to perturb current flow through the ground plane, resulting in a reduction in electromagnetic emissions. The invention is implemented with various types of magnets in different embodiments, including rare-earth magnets, AlNiCo magnets, Neodymium magnets, and flat-sheet magnets. A digital electronic device consistent with the present invention incorporates one or more magnets in proximity to a ground plane of a circuit board within the device, such devices including computers, set-top boxes, and personal digital assistants in various embodiments.